Humans have been applying tattoos to the skin for thousands of years. For example, the first recorded formula for mixing and applying tattoo ink dates back to the fifth century and is attributed to the Roman physician Aetius. Tattoo inks were derived from natural substances and comprised a suspension of pigmented particles in a liquid carrier. Applying tattoo ink with needles or similar instruments to the skin, where the ink remains permanently, produces tattoos. This technique introduces the pigment suspension through the skin by an alternating pressure-suction action caused by the elasticity of the skin in combination with the movement of the needle. Water and other carriers for the pigment introduced into the skin diffuse through the tissues and are absorbed. Once the skin has healed, most pigment particles remain in the interstitial space of the tissue. During the healing process, some particles of pigment are eliminated from the skin surface. After healing, the tattoo is made up of the remaining particles of pigment located in the dermis where they are engulfed by phagocytic skin cells or are retained in the extracellular matrix. See US Published application no. 2009/0311295 to Mathiowitz et al. Inks used for tattooing resist elimination due to their inertness and the relatively large size of the insoluble pigment particles. A tattoo produced in this manner will partially fade over time but will generally remain visible. Tattoos are used for a variety of reasons, primarily for ornamentation of the skin. See U.S. Pat. No. 6,013,122 to Klitzman & Koger.
Despite advances in methods of applying tattoo ink to the skin, such as the electric tattoo ink gun, tattoo inks in commercial use today are similar to those used centuries ago. Standard tattoo inks contain a pigment comprising metal salts dissolved in a carrier, usually ethanol or water, to disperse the pigment in the dermis. See U.S. Pat. No. 6,013,122. Thus a need exists for novel formulations of tattoo ink with improved properties.